The invention is related to the field of ring resonators, and in particular to Ge/Si resonator-based modulators for optical data communications in silicon photonics.
It is highly desired to have a field effect based modulator by using materials compatible with Si-CMOS platform. Ring resonators are gaining more and more interest due to its very small footprint (<a few tens μm), extremely high sensitivity to refractive index change, large extinction ratio and small power consumption. There are several reports on Si based ring modulators, where the refractive index change is induced by free carrier absorption.
It is well known that field effect devices are theoretically able to operate at the highest speed. Epitaxial SiGe on Si has been proposed for modulator devices by using Franz-Keldysh effects. However, there are several challenges needed to be solved in order to achieve workable ring modulator. First, Ge on Si is a high refractive index contrast system and its single mode dimension size is very small. Next, the index difference between Si and Ge is very large and it results in a very small coupling efficiency between Si waveguide and Ge (or SiGe) ring. Furthermore, depending on the operating composition of Si in SiGe, the Q-factor of Ge (or SiGe) ring could be low due to intrinsic absorption, which could result a low extinct ratio.